This application relates to optical dispersion, and more specifically, to techniques and systems for detecting and locating a fiber span with high polarization-mode dispersion in an optical fiber system.
Some optical transmission media such as optic fibers may be xe2x80x9cbirefringentxe2x80x9d, i.e., they have different refractive indices for light of different polarizations. Typical causes for such birefringence in certain fibers include, among others, imperfect circular core and unbalanced stress in a fiber along different transverse directions perpendicular to the fiber core. Such optic fibers are said to exhibit polarization-mode dispersion (xe2x80x9cPMDxe2x80x9d) because the different polarizations travel at different speeds. An optical pulse with optical components of different polarizations, therefore, may be broadened after propagation through such fibers.
This dispersion effect may be undesirable because the pulse broadening can limit the transmission bit rate, the transmission bandwidth, and other performance factors of an optical communication system. In fact, PMD is one of key limitations to the performance of some high-speed optical fiber communication systems at or above 10 Gbits/s due to the fiber birefringence. Fibers with significant PMD (e.g., about 1 to 100 ps/km) are used in various fiber networks, particularly in those that were deployed in 1980""s and early 1990""s. Hence, the compensation of PMD is desirable for high-speed transmission that uses those PMD systems.
The properties of PMD in a fiber system, however, need be measured and characterized in order to provide adequate compensation. A given fiber network or system, for example, may include fiber spans of varying degrees of PMD and only some fiber spans with high PMD contribute significantly to the total PMD. Hence, it may be desirable to identify and locate such a fiber span of high PMD in order to measure the properties of the PMD in that fiber span.
One device in this application includes a pump laser, an optical modulator, a photodetector, and a signal processor. The pump laser is operable to produce a pump beam with a power above a threshold value for a Stimulated Brillouin Scattering (SBS) process in a fiber span. The optical modulator is adapted to modulate the pump beam to produce a pulsed pump train in which each pulse has a pulse duration approximately equal to or longer than a time for the pump beam to travel a round trip in the fiber span. The photodetector is positioned to receive a SBS signal reflected from the fiber span to produce a detector output signal. The signal processor processes the detector output signal to determine a location of a fiber segment with a high PMD value in the fiber span.